Analog triggering circuitry is generally used in both analog and digital oscilloscopes. A conventional analog trigger circuit consists of signal conditioning networks, an analog comparator to compare the trigger signal to a predetermined trigger level, level controls, and slope validation circuitry. Such circuits must be designed carefully using analog design techniques that provide good gain and which minimize drift of performance characteristics over the operating temperature range and the aging cycle for the oscilloscope. Typically, potentiometers are provided to allow gain and offset adjustments during a calibration at the factory, which requires manual set-up and the use of test equipment. High cost components may be required to minimize the likelihood that the specified tolerances in operating characteristics will be exceeded during the life of the oscilloscope.
The exemplary trigger circuit of FIG. 1 illustrates some of the components involved in conventional triggering circuitry and the adjustments required. The trigger signal is received at an input BNC 21, which may be either an external trigger connector or may be connected to the input terminal for one of the channels to be displayed. The signal from the input is passed through an attenuator 22 and is then passed to an amplifier stage 24 which includes a gain adjustment potentiometer 25 and an offset adjustment potentiometer 26. At the factory, the potentiometers 25 and 26 are manually set by a technician to yield an output signal on the output line 28 of the amplifier 24 which has a desired gain level and response to a known input signal at the input 21 and which has a desired offset or null level. A signal on the line 28 is passed to a trigger comparator 30 which receives at its other input line 31 a voltage applied across a potentiometer 32. This potentiometer is set by the operator to yield the desired trigger level. The output of the comparator 30 is then passed to a slope logic circuit 34 which determines whether triggering is taking place on the proper slope of the signal, as selected by the operator, to validate when a proper triggering event has occurred. The output from the slope logic on a line 35 is a pulse which indicates the time of occurance of the valid trigger. This signal is then used by the oscilloscope to trigger a sweep across the face of the cathode ray tube (CRT) of the conventional oscilloscope or to initiate the acquisition and storage in a memory of digital data in a digital oscilloscope.
In addition to the necessary factory adjustments to properly calibrate the trigger circuit, recalibration may be necessary over a period of time as the equipment ages. For example, changes may occur in the characteristics of the attenuator 22, the amplifier 24, or the comparator 30 which may necessitate readjustment of the gain potentiometer 35 and the offset adjustment potentiometer 26. Such adjustment are generally made manually by a trained technician in the field, or by returning the oscilloscope to the manufacturer for factory calibration. The complexity of such calibration is compounded by the fact that the amplifier 24 is in the main signal path, which means that the amplifier must have proper gain adjustment over a relatively wide input range and over the wide range of frequencies which can be present in the input signal.